1. Field of the Invention
The present invention relates to a voltage regulator, and more specifically, to a voltage regulator having a soft-start function of suppressing an inrush current that occurs at a time of steep rise of power supply.
2. Description of the Related Art
FIG. 2 illustrates a block diagram of a voltage regulator. The voltage regulator includes an output transistor 1, a voltage dividing circuit 2, a reference voltage circuit 3, an error amplifier 4, and a soft-start circuit 5. At the time of steep rise of power supply, the soft-start circuit 5 controls a reference voltage Vref output from the reference voltage circuit 3 so that a reference voltage Vss may rise gently, to thereby suppress an inrush current.
The conventional soft-start circuit of the voltage regulator is formed of a circuit as illustrated in FIG. 3 (see, for example, Japanese Patent Application Laid-open No. 2008-109747).
The soft-start circuit 5 increases the reference voltage Vss gradually to a target value. The soft-start circuit 5 includes a digital circuit 71 for inputting a clock, a step circuit 72 for increasing the reference voltage Vref from 0 V to the target value in a stepwise manner, and a buffer circuit 73 for converting a change in step output of the step circuit 72 into a reference voltage Vss having a gentle gradient.
The conventional soft-start circuit, however, has the following problem.
When the reference voltage Vss is increased in a stepwise manner, as the reference voltage Vss becomes higher, the amount of the voltage increase per step becomes larger, resulting in an increased inrush current. On the other hand, in order to reduce the amount of the voltage increase per step, the number of stages of resistors needs to be larger, resulting in an increased area of the soft-start circuit.